1. Field of the Invention
This invention concerns a method for forming raised or reinforced regions on a rotationally molded part, such as a tank or vessel. More particularly, it is concerned with a process whereby a platform is provided within the mold and incorporated in the molded part to present a rib which structurally reinforces the part.
2. Description of the Prior Art
Rotational molding, as recognized by those in the art, is a process which is useful in forming parts from synthetic resin materials. The process of rotational molding is characterized by the provision of a mold separable into at least two mold sections, into which synthetic resin, such as polyethylene, is placed. Although heated liquid synthetic resin can be employed, the synthetic resin is typically provided in powdered or other solid form. By heating the mold within an oven-like chamber while the mold is rotated, preferably around more than one axis, the synthetic resin particles are distributed throughout the mold, thereafter melting to a gooey consistency when engaging the heated mold. The rotation of the mold results in the resin particles being substantially evenly distributed, with the resin filling in cavities in the mold and conforming to the interior configuration of the mold. Once the melting and distribution is complete, the mold is removed from the oven and cooled while rotation continues. Such cooling may be in ambient air temperature well below the melting point of the synthetic resin, or accelerated by the use of cooling water in hot environments.
Heretofore, molded articles have been provided with ridges or gussets through the use of kiss-offs or kiss-throughs. A kiss-off is that area of the molded part where two mold surfaces are separated by a sufficiently small distance that the resin collecting on the mold surfaces bridges the gap therebetween. A kiss-through is that area of the molded part where mold surfaces of two mold sections meet, thereby forming an opening in that area of the molded part where the mold surfaces met.
However, as increasing demands have been made on rotationally molded parts in regard to capacity, lifting strength, ability to resist compression, and resistance to impact, there has grown a need to provide for reinforcement in the form of ribs which may be formed in the parts to meet those demands. Such ribs would preferably be formed to provide additional material and strength, and could be molded into the part in areas where kiss-offs would be impractical.